1. Field of the Invention
The present invention relates generally to fuel supply systems for providing compressed natural gas fuel to a fleet of transit buses or the like.
2. Description of the Prior Art
As the search continues for cleaner burning fuels to reduce pollution in the nation""s cities, many city transit authorities are converting their bus fleets to run on compressed natural gas, commonly referred to as CNG.
Due to the high pressures at which the CNG must be stored, this presents unique engineering challenges for construction of the fuel systems.
Typically, the fuel on a CNG powered bus is stored in a series of elongated cylindrical tanks. These tanks may either be mounted below the floor of the bus or on top of the roof of the bus.
One example of a prior art roof mounted CNG fuel system for a transit bus is that manufactured by New Flyer. The New Flyer system utilizes a combination of four forward mounted and three rearward mounted Type 4 tank cylinders mounted on top of the bus. Type 4 tanks utilize a plastic liner with a carbon fiber overwrap. The tanks are supported by a pair of saddles and strap assemblies which typically support the tank at approximately xc2xc the distance from either end of the tank.
The New Flyer tanks are each supported on top of the bus by two saddles which are located immediately below their respective tanks. Two transverse manifold lines run across the roof of the bus between the forward and rearward tanks. Each tank includes a solenoid valve mounted in its end adjacent the tank, and a single piece of tubing connects each tank to one of the manifold lines, with the tank being both filled and discharged through that single piece of tubing. The tubing includes a horizontal portion, a 90xc2x0 upward bend, a vertical portion, a 90xc2x0 bend, a horizontal portion, another 90xc2x0 bend and then a vertical portion extending downward into a lateral port in the solenoid valve structure.
There are many shortcomings of prior art systems like that of New Flyer. One shortcoming is due to the manner in which the tanks are mounted on their saddle supports. The inner tanks of a set of four side-by-side tanks cannot be removed without first removing the outer tanks.
Another shortcoming is due to the mounting of the solenoid directly in the end of the tank. If the solenoid needs servicing, the entire tank must be emptied and purged so that the solenoid can be removed.
Still another shortcoming is the inadequacy of flexibility provided in the connecting conduit which can place excessive strain on the manifold line when the tank expands due to thermal expansion.
Furthermore, the tubing components for New Flyer are typically not pre-fabricated identical components.
Another prior art roof mounted CNG fuel supply system for transit buses is that in use by Orion Bus Industries. The Orion system includes four forward and four rearward roof mounted tank cylinders. The Orion tanks are Type 3 tanks. Type 3 tanks utilized an aluminum liner wrapped with a carbon fiber wrap. Type 3 tanks are mounted at their xe2x80x9cneckxe2x80x9d which is the end adjacent the manifold line. Substantially all thermal expansion of a Type 3 tank occurs at the end opposite the manifold line, thus flexing of the conduits connecting the manifold line to the tank are not a significant issue when utilizing Type 3 tanks.
The Orion system utilizes two parallel manifold lines which are not anchored to the bus, but which rather are supported from the tanks themselves. Thus, there is no longitudinal movement of the tank relative to the manifold line. The Orion tanks include a manual shutoff valve mounted in the end of the tank. A check valve is mounted on one side of the shutoff valve and a solenoid valve is mounted on the other side of the shutoff valve. An S-shaped tube connects the manifold line to the check valve. A return tube connects the solenoid valve to the manifold line. To the extent there is flexibility built into the tubing lines of the Orion system, it is provided simply to accommodate flexing of the bus itself, and not because of any thermal expansion of the tank end relative to the manifold line. Many of the Orion tubing components are pre-fabricated identical components, but they are not designed for use with Type 4 tanks.
Thus, it is seen that there is a need for improvement in CNG fuel supply systems, and particularly for improvement in the design and construction of the manifold lines and tubing connecting the manifold lines to the tanks when utilizing Type 4 tanks which exhibit substantial thermal expansion relative to the manifold lines.
There is particularly a need for improvements in systems for utilizing Type 4 tanks with a fleet of buses, wherein the efficiency of maintenance of the overall fleet is improved by providing the ability to interchange tubing components between buses and by providing the ability to remove any selected tank for service without removing the other tanks.
A CNG fuel supply system is provided for using a plurality of Type 4 cylindrical tanks mounted on the roof of a bus, the bus being one of a fleet of such buses. A manifold line extends to the roof of the bus and is anchored against movement relative to the length of the bus. A shutoff valve is associated with each of the Type 4 tanks, the shutoff valve being mounted on an end of its respective tank adjacent the manifold line and having two ports which communicate with the tank when the shutoff valve is open. Each of the tanks has associated therewith an inlet tubing piece connecting the manifold line to one of the two ports on the shutoff valve, and an outlet tubing piece connecting the manifold line to the other of the two ports on the shutoff valve. The tubing pieces are substantially identical to tubing pieces for some of the other tanks on the bus and to the similar tubing pieces on other buses, and are pre-fabricated machine bent tubing formed to specified tolerances so that at least some of the tubing pieces are interchangeable.
This provides a system wherein the tubing pieces are interchangeable between buses to allow replacements to be made with pre-fabricated tubing pieces kept in a maintenance inventory.
Many of the tubing pieces include a flexible portion to accommodate thermal expansion and contraction of the Type 4 tanks, which flexible portion includes a continuous 180xc2x0 bend having a radius of at least xc2xd inches to the center line of the tubing.
The tank system may initially include four rearward tanks and two forward tanks, and the manifold line is designed to allow the addition of two additional forward tanks using tubing pieces identical to the tubing pieces for the rearward tanks.
Each of the tanks is mounted upon the roof of its respective bus so that the tank can be removed without removing any of the other tanks.
An improved fill block may also be provided for connecting the manifold line to a source of CNG to fill the system.
It is therefore, a general object of the present invention to provide improved CNG fuel systems for utilizing Type 4 tanks mounted on the roof of a bus.
Another object of the present invention is the provision of a fleet of buses utilizing such fuel systems, wherein the maintenance of the bus fleet is enhanced by the design of the system for improved serviceability of the system and for maintenance of the system utilizing replaceable pre-fabricated machine bent tubing components.